Changed FacebookBase::errorLog to a non-static method so that it may be overridden by subclasses.
All references have been updated from `self::errorLog(...` to `$this->errorLog(...`
The unit test `testErrorLogOverride` has been added to ensure that error logging behavior can be overridden by subclass.
Merge pull request #1 from tagged/override_errorLog

Changed FacebookBase::errorLog to a non-static method